In marine-seismic investigations, seismic signals are generated, for example, by air pulse generators or blasting charges dragged behind a measuring ship. Sea measuring cables, called streamers, serve for receiving sound signals in the body of water. Measuring cells which react to pressure changes, called hydrophones, are arranged in each streamer and the measuring cells register the seismic reflection signals from the lower sea floor.
Streamers of this type are usually dragged by the measuring ship, the measuring cable generally consisting of a plurality of sections which are connected mechanically and electrically by quick-fitting couplings. The overall length of a streamer can be several kilometers. The tensile force acting on the streamer is transmitted from one quick-fitting coupling to the next quick-fitting coupling by hauling cables such as steel cables. Shaped pieces which are spaced apart in the longitudinal direction are provided to maintain the spacing of the hauling cables and to provide for transverse stability of the streamer. This arrangement forms a skeleton structure which absorbs the forces acting on the streamer. The skeleton structure is drawn into a tube for protection against sea water. To facilitate this assembly, the shaped pieces have an outside diameter which is slightly smaller than the inside diameter of the tube.
Arranged between the shaped pieces are the hydrophones whose wiring is passed through bores in the shaped pieces. In order to adapt the density of the streamer to the density of the sea water, the streamer is filled with liquid, in particular oil.
A continuous column of liquid in the tube is required so that the tube can be filled with liquid.
The shaped pieces have an annular gap from the streamer tube and generally a bore of large passage width arranged in the longitudinal direction so that the oil filling in the streamer can flow freely to a great extent. In this case, however, it is disadvantageous that the oil can move in a substantially unobstructed manner in each individual streamer section, even during the measuring run. In the event of longitudinal vibrations of the streamer during a measuring run arising, for example, from resonances caused by the rotational speed of the ship's shaft or vibrations of sea waves, the column of oil enclosed in the streamer is thereby moved relative to the skeleton structure. At the ends of the column of oil, for example at the couplings of the individual streamer sections, the undisturbed pressure prevailing at the site of the receiver is alternately increased and reduced. This relative movement is converted into pressure waves. The pressure waves are acoustic interference signals which falsify or are superimposed on the useful seismic signals.
A marine-seismic streamer having a liquid-filled tube is known from EP 171,936, in which, in the longitudinal direction, hauling cables, shaped pieces which are spaced apart in the longitudinal direction, and hydrophones as well as flow-obstructing openings in the shaped parts are provided for dividing the streamer into sections.
The size of the openings used allows only very slow pressure compensation between the sections. Since the outside diameter of the shaped pieces corresponds to the inside diameter of the tube, the tube is constricted under tensile force between each pair of successive shaped pieces, resulting in an undulating outer shape of the tube overall in operation.
DE 2,941,028 A1 shows a streamer having an additional inside tube in which hydrophones are received via an anchorage in open-pore foamed material holders. The inside tube itself is centered in the outside tube via spacers. The construction of this streamer is very complex, in particular when considering the arrangement of the wiring of the hydrophones which are passed in each case into the outside tube through the wall of the inside tube. Exchanging hydrophones is therefore only possible at unacceptable expense.
Additionally, due to the use of two coaxial tubes, differential vibrations occur which can only be eliminated by complex measures.